# std::allocator_arg, std::allocator_arg_t

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< cpp‎ | memory

Defined in header `<memory>` |  |
struct allocator_arg_t { explicit allocator_arg_t() = default; }; | (1) | (since C++11)
constexpr std::allocator_arg_t allocator_arg {}; | (2) | (since C++11) (inline since C++17)

1) `std::allocator_arg_t` is an empty class type used to disambiguate the overloads of constructors and member functions of allocator-aware objects, including std::tuple, std::function, std::packaged_task,(until C++17) and std::promise.
2) `std::allocator_arg` is an instance of (1) that can be passed to the constructors and member functions of such permitted types.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2510 | C++11 | the default constructor was non-explicit, which could lead to ambiguity | made explicit

### See also

uses_allocator (C++11) | checks if the specified type supports uses-allocator construction (class template)

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